The recent discovery of high critical temperature superconducting materials has created interest in the use of these materials in microwave devices. The HTSC metal cuprate materials cannot readily be produced in bulk with geometries and properties suitable for microwave devices. Presently, devices based on the HTSC materials are fabricated by formation of thin films on substrates by techniques similar to those used to fabricate semiconductor devices. To be useful in a microwave device the HTSC film must be grown on a substrate having low dielectric losses at high frequencies.
Silicon, a readily used substrate for semiconductor devices, cannot be used with the HTSC cuprate materials since at the temperature prevalent during deposition the cuprate reactants readily react with silicon. Sapphire (Al.sub.2 O.sub.3) would appear to be an excellent substrate since it has a very low dielectric loss and is a strong, low cost, highly crystalline material available commercially in large sizes. Epitaxial films of a HTSC material such as YBa.sub.2 Cu.sub.3 O.sub.7 can be grown on sapphire substrates. However, the optimum window of substrate temperatures during film deposition is relatively narrow [4]. At high temperatures, above about 700K, the YBa.sub.2 Cu.sub.3 O.sub.7 thin film reacts with the sapphire, especially the Ba atoms. At low temperature, below about 650K, it is very difficult to produce thin YBa.sub.2 Cu.sub.3 O.sub.7 films having good epitaxy.
There are other substrate materials that provide epitaxial growth of thin HTSC films with high superconducting transition temperatures and low rf surface resistance, such as strontium titanate (SrTiO.sub.3), lanthanum aluminate (LaAlO.sub.3), magnesium oxide (MgO) and yttria stabilized zirconia (YSZ). However, thick substrates of some of these materials, e.g. strontium titanate, exhibit too high an rf loss or do not have high enough mechanical strength to act as a substrate for large area microwave devices. Some of these substrates are not available in large sizes and/or are only available at high cost.
However, thin films of these epitaxial materials would not exhibit a high dielectric loss and would be useful as a buffer layer between the HTSC film and the sapphire substrate if they provided an epitaxial surface for the HTSC film and were stable and non-reactive with the HTSC film and the substrate.